1. Field of the Invention
The present invention relates generally to cryogenic liquid delivery systems and more particularly to managed dosing systems for injecting metered droplets of liquid nitrogen into beverage, food or other product containers as they move along high-speed production lines before being sealed.
2. Description of the Related Art
With thin walled containers, especially thin walled metal cans and plastic bottles, it has been found useful to stiffen them after filling, but prior to further processing, such as before labeling, shipping and handling to prevent subsequent container damage. To achieve such stiffening, a cryogen such as liquid nitrogen may be injected just prior to sealing. Injected as droplets, the liquid cryogen undergoes phase change to a gas, increasing the pressure inside the container, the increased pressure acting to stiffen the container walls.
Typically, the liquid cryogen drops or droplets, once injected, will coalesce as they sit on the container contents, the vaporization process taking anywhere from 5-15 seconds. Accordingly, the time between injection and container closure must be kept short. It is to be appreciated the exact time of vaporization may vary depending upon the size of the injected droplet, and the temperature of the container contents. The resulting pressure within the container will similarly be a function of the size of the injected drop, the free space to be filled, and the time between droplet injection and container closure.
Since the liquid nitrogen begins to immediately vaporize upon being dispensed, it is desirable to cap or close the container as soon as possible. Preferably, injection should occur immediately upstream of the closure station. However, because of the physical layout and limitations of conveyor systems used to bring containers to a capping or closure station, the size of the liquid delivery system head, and the configuration of the closure station itself, it is presently necessary to inject the liquid nitrogen a distance upstream of the point of closure.
Typical of liquid injection delivery systems developed for injection of small amounts of nitrogen into containers as they pass along an assembly line are those sold by VBS International, Inc. of Campbell, Calif., under the trade names LCI-300, 400, and 2000M. See also U.S. Pat. No. 6,182,715 to Alex R. Ziegler, et al, which patent is incorporated herein by reference in its entirety.
In these systems, a stream of liquid cryogen droplets is dispensed vertically into a moving container. In so doing, the force of injection can cause the droplets to substantially penetrate the surface of the container contents. The force of impact can result in splash-back of the contents onto the dosing head, where the splashed liquid may accumulate and later interfere with the operation of the dosing head itself.
Conveyer systems are run at fairly high speeds where containers pass by fixed stations at the rate of 500 units per minute or more. In fact, some processing conveyor lines run to speeds in excess of 1500 to 2000 containers per minute. At lower speeds, e.g. 500 units per minute, the liquid nitrogen feed systems of the referenced prior art perform well. However, at higher line speeds, the dispensing assemblies must operate at higher frequencies. Pneumatically driven valves such as those used in the dispensing systems of VBS to meter dose amounts produce heat proportional to their speed of operation, the pressure of the gas source, and frictional loses. As a result, heat tends to build up as the pneumatic valve is more rapidly cycled.
To date, it has been problematic to operate at the higher conveyor speeds of 1000 to 2000 containers per minute. In fact at such operational speeds, the pneumatic system gets hot to the touch (140° F.-160° F. and above), seals may fail and the unit burn out over the course of a day. Further, these delivery systems frequently are installed in assembly line areas where ambient temperatures may easily exceed 40° C., reducing the potential for effective ambient cooling.
With such high speed lines where containers pass a fill point at the rate of upwards of 1000 to 2000 units per minute, the residence time at the liquid injection station also becomes a factor, with the time allowed for fill becoming shorter than the time required for delivery of the dispensed liquid dose stream. This mismatch can result in a good portion of the injected dose missing the container opening, and thus lost to the atmosphere by vaporization. As a further result, maintenance of dose accuracy and repeatability can be lost.
There thus remains a need to develop delivery systems which are less prone to clogging through splash-back, and able to more accurately and efficiently deliver a measured dose of cryogen to a container to be pressurized. There also remains the need to shorten the dispensing cycle time of existing liquid delivery systems so as to match the higher speeds of current conveyor systems. So too, there remains a need for these systems to be able to operate in harsher temperature environments, such that the surrounding ambient will have little to no effect on operations.